How does the melting behaviour vary between amorphous and semi-crystalling polymers?
Semi-crystalline polymers have a clear melting point
Amorphous polymers have no melting point, but a glass transition temperature
What is the behaviour of a Polymer above the Glass transition Temperature Tg and below Tg?
Above: elastic rubbery
below: hard and brittle
How is it possible for the semicrystalline Polymer to have melting point and Tg?
There are always amorphous parts in between the semicrystalline structures.
How does the Temperature behave at melting, when a constant heating rate is applied?
The temperature rise stops until all the Polymer is melted.
How does the Temperature behave around the Glas transitioning temperature, when a constant heating rate is applied?
The temperature increase is continual.
What is the rule of thumb for the relation between Tg and Tm in Polymers?
What are polymeric “crystals” called?
Spherolites
What is the effect of increased supercooling on the crystallization speed?
It increases the crystallization speed, by increasing the driving Force.
How does the rate of crystallization behave below Tg?
It becomes almost 0 due to the disabillity of the molecules to move.
What are the two variants to induce crystallinity?
ordering by slow cooling below the melting point
ordering by stretching
Which molecular properties increase the crystallinity?
Narrow molecular weight distribution
large molecular weight
linear polymer chains
regularity of the tacticity
Name the components and the significance of the A,B,C curve
A=amorphous
B=amorphous (border to crystalline)
C=crystalline
GIve the Avrami equation and its meaning.
The crystallinity degree dependant on the time.
What does the Thompson equation describe? Give the Thompson equation.
The deviation of the melting point in non ideal conditions
Which stages in crystallization are described by the primary and secondary crystallization?
Primary crystallization until the spherolites meet eachother secondary (slower) crystallization afterwards.
Describe the difference in specific volume change between crystallization and amorphous solidification.
How can the crystallinity content be evaluated in the DSC analysis? (give formula)
Hm=melting enthalphy
Hc=crystallization enthalphy
Ho=melting enthalpy of sample with 100% crystallinity
How can the crystallinity content be evaluated by the Archimedis analysis? (give formula)
How can the crystallinity content be evaluated by the wide-angle X-ray sattering (WAXS) analysis? (give formula)
Describe the principle of Differential Scanning Calometry.
Two crucibles are compared with eachother. One is empty one holds the sample. At a fixed increase in temperature the use of heat is compared. The output is a compared diagramm of the heat flow (Q/t) over temperature.
How is the glas transition temperature determined in DSC?
middle of sloping curve
higher after because heat capacity increases
What is the meaning of the following peak in DSC?
Crystallization, which is an exothermic reaction, after obtaining enough mobility to order.
Give the meaning of theses peaks
Tg:glass transition
Tc:crystallification
Tm:melting and dissociation of the crystal
Which order f heating is normally conducted in DSC?
1st heating cycle
controlled cooling
2nd heating cycle (to obtain process independant properties)
What is the difference between LDPE, HDPE and UHMWPE?
LDPE: most bracnched and therefore lowest density
HDPE: minimal branching
UHMWPE: around 100 times molecular weight with molecules in one direction
better comperatively mechanical properties
What is described by the term curing? How does the energy behave in the process?
the toughening of polymeric materials by increasing their cross links.
It is an exothermal process
What is indicated by Mn, Mw and Mz in the context of molecular weight? Give the formulas.
(for discontinous:)
What is mesured by the Polydispersity index and how is it calculated?
The spread of the molecular weight, by relating the most common molecular weight with the average molecular weight.
What are the types of fluids and their relation between shear stress and shear rate?
newtonian fluids
Bingham fluids
Pseudoplastic fluids
Dilatant fluids
What is described by the Flory-Fox equation? Give the equation.
The relation between molecular weight and glass transion temperature
Tg infinty: theroetical value for endless molecules
K:empirical parameter
What is the effect of molecular weight on the crystallinity?
High Molecular weiht decreases the crystallinity
What is the Relaxation modulus and how is it affected by the molecular weight?
after a elongation the stress decreases after initial peak due to viscoelasticity (E=G)
What is the melt flow index
The inverse of viscocity
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